Nowadays information technology is confronted with an increasing amount of data, due to more complex or multimedia applications. Accordingly, removable data storage devices with a high storage capacity are needed, e.g. for high resolution movies or video games. Well at the beginning of information technology, magnetic storage devices were favored, while nowadays, optical storage media such as CD (Compact Disk), DVD (Digital Versatile Disk) or BD (Blu-Ray Disk) are dominating the market for removable data storage media.
Optical data storage is generally limited by the optical resolution of the read/write-system. Straightforward methods of increasing the optical resolution involve widening of the focused beam and opening angle, i.e. the numerical aperture NA, at the costs of lens complexity. Further approaches are narrowing the allowable tilt margins for the optical storage media or reducing the wavelength of the scanning laser into the blue or near-UV range. A different approach for reducing the focus spot size in an optical data storage system is using near-field optics with a high numerical aperture (NA>1). This high numerical aperture is generally achieved by help of a solid immersion lens (SIL). While conventional systems like CD, DVD or BD operate in the optical far-field regime, which is described by classical optics, the aforementioned new systems work in the optical near-field regime, which is described by near-field optics. For conventional systems the working distance, i.e. the air gap between the surface of the optical storage medium and the first optical surface of the read/write-head, usually the objective lens, is in the scale of 100 μm. In contrast, systems making use of near-field optics need a very small working distance or air gap, which is in the scale of 50 nm. An optical storage system for recording and/or reading making use of near-field optics is disclosed in WO 2005/104109 A1. The low working distance between the surface of the storage medium and the surface of the objective lens is one of the main challenges of near-field technology. The small working distance requires setting strict specifications on the tilt and vertical deviation limits for the related optical storage medium e.g. a disc. Even if the disc specifications only allow vertical run out values which are several times smaller than for a current Blu-Ray Discs, e.g. 20 μm instead of 100 μm, it is still difficult for the read/write-head of an optical pick up to approach the disc surface and close the focus loop without head-disc contact or head crash.
A straight forward approach is to start the read/write operation from a lead-in area where tilt and deviation are usually lower compared to outer areas of the storage medium. However, for a near-field optical storage system it is desirable to reach outer areas of the storage medium without continuous focus operation at the aforementioned low working distance of approx. 50 nm.
Also, JP 11-259897 discloses an optical pickup capable of recording and reproduction for both high-density and low-density optical recording media. The pickup has a first objective for low-density media, which converges collimated light, a light shield which cuts off part of the light passing through the first objective, and an optical element consisting of a second objective and a solid immersion lens for high-density media. For low-density media the head is put close to the light shield by a slider. For high-density media the head is put adjacent to the recording medium.
US 2008/0198728 discloses an optical disk drive comprising an objective lens, a solid immersion lens, and an aperture element for collecting a part of a light beam reflected by an optical recording medium, corresponding to the effective aperture number of the objective lens and the solid immersion lens of less than 1 A speed-generating circuit reduces a speed of approach of the solid immersion lens to the recording medium in accordance with the level of a signal detected by a detecting element. A driver circuit drives the objective lens and the solid immersion lens in accordance with the output from the speed-generating circuit. In this way it is assured that the solid immersion lens does not get into contact with the optical recording medium.
US 2008/0089208 discloses a near field optical scanning device, which is adapted to bring a lens from a remote position to a near field position relative to the surface of an optical recording medium. Image processing of aperture pupil images indicating the size of a gap between a solid immersion lens and the surface of the optical recording medium is used for this purpose. Image analysis of the aperture pupil image allows to derive a control signal for an approach procedure for air gap distances in a range of micrometers. This allows for a fast, efficient, accurate and reliable approach procedure.